reserve k,j,n for Nat,
  r for Real;
reserve x,x1,x2,y for Element of REAL n;
reserve f for real-valued FinSequence;

theorem Th10:
  |.x1 - x2.| <= |.x1.| + |.x2.|
proof
  |.x1 - x2.| <= |.x1.| + |.-x2.| by Th9;
  hence thesis by Th7;
end;
